A transimpedance amplifier (“TIA”) is commonly used to convert an input current signal into an output voltage signal proportional to the input current signal. The input current signal is generally provided by a photo detector such as a photo diode, which converts optical energy transmitted from an optical source into electrical energy in the form of an electrical current. A TIA is often implemented by providing a feedback resistor coupled between an input and an output of an operational amplifier. However, since the optical source may operate in an unstable light environment, the electrical current provided by a photo detector is subject to change as the optical energy changes. As an example, the electrical current may range from 1 microampere to 1 milliampere. Given a resistor of 100 kilo-ohms used in the TIA, the output voltage may range from 1 millivolt to 0.1 volt to 100 volts, which is not acceptable. Moreover, the resistance of a feedback resistor is related to the sensitivity of a TIA. It is generally desirable to use a feedback resistor having a resistance value as great as possible if the bandwidth of the TIA permits. A gain-controlled TIA has been therefore developed to allow a normal gain in response to a normal input current, and a controlled gain in response to an input current that will result in an unacceptable gain region.
FIG. 1 is a schematic diagram of a gain-controlled TIA circuit 10. Referring to FIG. 1, gain-controlled circuit 10 includes an operational amplifier 12, a resistor 14 and an n-channel metal-oxide-semiconductor (“NMOS”) transistor 16. Gain-controlled TIA circuit 10 includes an input terminal (not numbered) to receive an input current IIN, and an output terminal (not numbered) to provide an output voltage VOUT. In operation, in response to a normal input current IIN, NMOS transistor 16 is turned off, gain-controlled TIA circuit 10 provides VOUT equal to −IIN×R, where R is the resistance value of resistor 14. When input current IIN exceeds an acceptable region, NMOS transistor 16 turns on, which functions to serve as a variable resistor. Gain-controlled TIA circuit 10 provides VOUT equal to −IIN×(R//R′), where R′ is the resistance value of NMOS transistor 16. Gain-controlled TIA circuit 10 therefore provides a gain of R in response to a normal input current, and a gain of R//R′ equal to (R×R′)/(R+R′) in response to a relatively large input current.
In gain-controlled TIA circuit 10, the variable resistance R′ decreases as a drain-to-source current IDS of NMOS transistor 16 increases, which in turn results from an increase in a gate-to-source voltage VGS of NMOS transistor 16. Accordingly, the variable resistance R′ decreases as a voltage VG applied to a gate of NMOS transistor 16 increases. It is important to control the bias voltage VG in order to provide a controlled gain for TIA circuit 10. An example of a gain-controlled TIA circuit is disclosed in U.S. Pat. No. 6,593,810 (hereinafter the '810 patent) to Yoon, entitled “2.5 Gigabits-per-Second Transimpedance Amplifier.” In the '810 patent, a resistor divider network 47 including resistors R1 and R2 is coupled between an output 90 of a replica biasing stage 40 and ground. Resistor divider network 47 provides a scaled output voltage V at a terminal 90 disposed between resistors R1 and R2, which is proportional to a voltage level VO at output 90 of replica biasing stage 40. The scaled output voltage V is compared with a voltage level at an output 80 of a gain stage 30 to determine whether to turn on a degenerative feedback element 15. However, the voltage level VO may be deviated due to the loading resistors R1 and R2. As a result, the '810 patent may not provide a fixed scaled output voltage V, which adversely affects the performance of a TIA system.